1. Field of the Invention
This invention relates to a method for the reduction of carbon dioxide into carbon monoxide, a catalyst for use in the reduction of carbon dioxide, and a method for the production of the catalyst.
In recent years, the earth's warming due to an increase in the carbon dioxide concentration of the atmospheric air has been arousing the public anxiety. Though the carbon dioxide has no toxicity in itself, it is now discharged in a colossal amount of about 20 billion tons annually into the atmospheric air. In the circumstances, the desirability of preventing a further increase in the carbon dioxide concentration in the atmospheric air has been finding widespread recognition. This prevention dictates an extreme decrease in the consumption of energy and an increase in the reclamation of produced carbon dioxide. The carbon monoxide which is obtained by reducing carbon dioxide can be utilized as a raw material for products of synthesis and as a fuel. This invention concerns the reduction of carbon dioxide mentioned above.
2. Description of the Prior Art
The method of reduction and fixation of carbon dioxide which resorts to catalytic hydrogenation of the carbon dioxide, i.e. the reduction of the carbon dioxide by reaction with hydrogen, possesses a prominent capacity for reducing and fixing the carbon dioxide per unit time per unit surface area and allows treatment of the carbon dioxide in a large amount as compared with the method of photochemical reaction, the method of electrochemical reaction, the method of polymer synthesis and the method of organic synthesis. It further enjoys the advantage that it allows application thereto of the technique of hydrocarbon synthesis by the Fischer-Tropsch process and, by reason of the vapor-phase reaction in mode of operation, permits the product of the reaction to be easily separated. Heretofore, for the reduction and fixation of carbon dioxide by the catalytic hydrogenation, the method which uses a catalyst of such noble metal as ruthenium or rhodium has been studied [as reported by F. Solymosi and A. Erdohelyi, J. Mol. Catal., Vol. 8, 471 (1980)].
This method, however, is disadvantageous in (1) that the catalyst to be used therein is expensive and is easily poisoned by such sulfur compounds as hydrogen sulfide and sulfur dioxide and is deprived quickly of its catalytic activity, (2) that this reaction which reduces carbon dioxide into methane is an exothermic reaction whose product has a lower energy than the raw material and, therefore, suffers from poor energy yield, (3) that more often than not the reaction fails to proceed smoothly unless it is carried out under high pressure and (4) that since the reaction is generally carried out at an elevated temperature and this elevation of temperature necessitates use of a fossil fuel, the reaction brings about no substantial repression of the occurrence of carbon dioxide.
In the circumstances, therefore, a desire is expressed strongly for a method which is capable of reducing carbon dioxide into carbon monoxide economically by a simple procedure with a notably small consumption of energy.
The present inventors have conducted various studies with a view to attaining this desire.
They have taken notice of the fact that tungsten sulfide which is used as a catalyst for hydrodesulfurization and a hydrogenating catalyst for ethylene hydrocarbons, for example, features the quality of defying the poisoning action of such sulfur compounds as hydrogen sulfide and sulfur dioxide gas, producing no toxic action, exhibiting strong resistance to acids, and enjoying durability, that the necessity for using an expensive noble metal can be obviated and that tungsten sulfide is black and amply absorbs the sunbeams. They have consequently conceived an idea that the use of tungsten sulfide results in accomplishment of the desire mentioned above. This invention has been perfected as a result.